A typical wireless communication system comprises a plurality of wireless communications devices exchanging data or voice data with each other. One example of a wireless communication system is a high frequency (HF) wireless communication system. The typical HF wireless communication system provides several benefits. For example, the HF wireless communication system offers potential worldwide communication capabilities with little to no infrastructure. Indeed, HF communication is popular with many amateur (HAM) radio operators, permitting one operator to readily contact another operator on another continent. The long range of HF wireless communication systems is the result of the desirable propagation characteristics of HF waves, resulting from their refraction by the Earth's ionosphere. However, HF communications can be detrimentally impacted by unfavorable ionospheric conditions. The conditions that may affect HF communications include, for example, sunlight/darkness at site of transmission and reception, season, solar sunspot cycle, solar activity, and polar aurora. In particular, these conditions can cause only some HF frequencies to be usable on a particular link; which frequencies are usable at any given time may be difficult to predict. Accordingly, the user may be forced to manually cycle through several frequencies to find a channel suitable for transmission.
A typical approach to this drawback in HF communication systems is automatic link establishment (ALE) methods. The ALE approach may typically include the transmitter device continuously scanning all available frequencies to determine corresponding quality-of-service (QoS) values for each frequency. Helpfully, the user of the HF communication system need not manually scan and evaluate the available frequencies. When a communication is initiated, the transmitter device selects the best available frequency for the desired transmission path.
Potential drawbacks to the ALE method may include lengthy link establishment times. For example, U.S. Pat. No. 5,930,685 to Straub discloses a method of ALE between two devices. This method inserts a 24-bit word in the call transmission for speeding up the link establishment. Moreover, even with ALE, the design of the typical HF wireless communication system may suffer from limited bandwidth, since existing ALE techniques lack capabilities to evaluate channels wider than 3 kHz, and to coordinate bandwidth selection between calling and called stations.
Another potential drawback to HF communications is difficulty encountered when the transmitting device has a low power transmitter, and/or a disadvantaged antenna. In these applications, the signal-to-noise ratio (SNR) at the receiver device may be quite poor, and the user may be limited to short messages that contain only digital data such as text characters, possibly having no latency requirements and low bandwidth. Effective use of the channel for communications may be limited to short messages containing textual data only, delivered on a best-effort basis without acknowledgement or Automatic Repeat reQuest (ARQ).
An approach by the present application's assignee to such a drawback has been to provide a spread HF 3 kHz waveform (Chamberlain, M, Furman, W. and Leiby, E, “A Scaleable Burst HF Modem”, Proceedings of HF98, The Nordic Shortwave Conference). The waveform uses orthogonal Walsh symbol modulation combined with M-phase shift keying (PSK) or Gaussian minimum shift keying (GMSK).
An approach to this drawback may comprise the phase shift keying, 31 Baud (PSK31) protocol. The PSK31 protocol comprises coupling a typical personal computer to a transceiver, providing an audio tone from the computer, and outputting it to the transceiver. The computer switches the polarity of the signal used to key the computer's sound card, which may provide both amplitude and phase modulation of the carrier signal.